feat(context): consolidate message UI overrides on the MessageUI slot

[oliverlaz]

Goal

REACT-1027

ComponentContext.Message was deprecated in favour of MessageUI during v14 but never removed. Removing it surfaced four more ways to reach the same component — a Message prop on Message, MessageList, VirtualizedMessageList and Thread — now all collapsed onto the MessageUI slot.

BREAKING CHANGES:

• ComponentContext.Message removed → use the MessageUI slot.

• The Message prop is removed from Message, MessageList, VirtualizedMessageList and Thread, and therefore from additionalMessageListProps, additionalVirtualizedMessageListProps and additionalParentMessageProps. To replace a per-component prop, scope the slot to that subtree:

  // before
  <Thread Message={CustomThreadMessage} />
  
  // after
  <WithComponents overrides={{ MessageUI: CustomThreadMessage }}>
    <Thread />
  </WithComponents>

• areMessagePropsEqual no longer compares the message UI component — it comes from context, and context updates re-render consumers regardless of React.memo.

VirtualMessage is unchanged and still wins inside VirtualizedMessageList, but now only within that list. Full migration detail: ai-docs/ai-migration-v14-v15.md.

Implementation details

MessageProps.Message was the transport VirtualizedMessageList and Thread used to inject their resolved component, so deleting it naively would have silently broken the VirtualMessage slot. Instead, VirtualizedMessageList applies VirtualMessage to its own subtree's ComponentContext — wrapping only when the slot is set — which also retires VirtuosoContext.Message. Thread's three-step resolution collapses entirely; context already carries the component. WithComponents now memoizes its merged override map: required, not cosmetic, since an unmemoized provider on the list's render path would defeat per-message memoization.

VirtualMessage had no test coverage, which is what made this risky. Added three tests — precedence, subtree scoping, unset fallback — and the precedence one fails on release-v15. They assert what useComponentContext() resolves to rather than inspecting rendered items, because Virtuoso renders zero items under jsdom and an item-level assertion would pass either way.

Verified: tsc -p tsconfig.lib.json --noEmit clean · yarn test 231 files, 2828 passed / 1 skipped · yarn lint-fix clean. Each documented failure mode was compiled to confirm it errors (TS2322 for props, TS2561 for override keys).

Unrelated change: the v14 → v15 guide also gains a section on the context-hook componentName removal from #3267, included at the author's request rather than split out.

UI Changes

None — override plumbing only; the default message UI and every existing MessageUI / VirtualMessage override render exactly as before.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-v15@9194215). Learn more about missing BASE report.

Additional details and impacted files

@@              Coverage Diff               @@
##             release-v15    #3268   +/-   ##
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  Coverage               ?   84.46%           
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  Files                  ?      526           
  Lines                  ?    15939           
  Branches               ?     5108           
==============================================
  Hits                   ?    13463           
  Misses                 ?     2476           
  Partials               ?        0           

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